1. Field of the Invention
This invention relates to a centrifugal type friction clutch for a transmission which permits power to be transmitted in either direction, for example, power transmission from an input member to an output member (engine drive) and power transmission from the output member to the input member (engine brake) of a motor vehicle.
2. Description of the Prior Art
It is known in the art that slipping of a clutch is effective for reducing vibrations and noises which occur a high load in a motor vehicle with a centrifugal type friction clutch. Clutch slippage reduces the torsional vibrations caused by variations in torque which are applied to the drive system of the vehicle by the reciprocating mass of a driving piston as a result of detonations (knocking) in the engine cylinder. In this connection, U.S. Pat. No. 4,305,493 proposes a centrifugal type friction clutch which includes, as shown in FIGS. 1 to 5, slidable friction shoes 40 each having an engaging portion and C-shaped opposite end portions which are bent inward, a follower member 50 held by guides at its opposite ends, a leaf-like load spring 60 provided between the follower member 50 and friction shoe 40 and a retractor spring 80 positioned between the load spring 60 and follower member 50 and engaged at opposite ends with paired projections 70 provided on the disc 10.
This centrifugal friction clutch operates in the following manner:
(a) Low-speed non-operating range (FIG. 1)
The friction shoe 40 against the paired projections 70, and forms at one end of the load spring 60, a leading contact portion 40A in power transmission in driving state (hereinafter referred to as "drive power transmission" for brevity) and at the other end a leading contact portion 40B in power transmission in driven state (hereinafter referred to as "driven power transmission).
(b) Low-speed operating range (FIG. 2)
The engaging portion of the friction shoe 40 is displaced radially outward and held in engagement with the opposing friction surface of the drum 20.
(c) Medium- and low-speed centrifugal clutch range (FIG. 3)
The friction shoe 40, follower member 50 and retractor spring 80 are urged toward the drum 20, and the friction shoe 40 and follower member 50 are separated from each other.
(d) Medium- and high-speed torque limiter range (FIG. 4)
Opposite ends of the retractor spring 80 are abutted against the follower member 50, and the friction shoe 40 and follower member 50 are separated from each other.
(e) High-speed range (FIG. 5)
The load limiting spring 60 and retractor spring 80 are separated from each other, and the engaging portion of the friction shoe 40 is held in engagement with the drum 20 solely by the centrifugal force.
When a centrifugal friction clutch of such a construction is used on a motor vehicle, it is possible to slip the clutch only in drive power transmission upon application of a large load since the friction shoe 40 serves as a leading shoe in both drive and driven states. However, although the clutch slip is necessary at a smaller load in a driven power transmission than in a drive power transmission, it has been difficult to comply to this requirement, failing to reduce the torsional vibrations during an engine-braked cruising operation for suppression of vibrations and noises of a vehicle.